{"id":10637,"date":"2017-07-14t15:34:32","date_gmt":"2017-07-14t22:34:32","guid":{"rendered":"\/\/www.catharsisit.com\/hs\/?p=10637"},"modified":"2017-07-14t15:34:32","modified_gmt":"2017-07-14t22:34:32","slug":"ap-calculus-review-implicit-variation","status":"publish","type":"post","link":"\/\/www.catharsisit.com\/hs\/ap\/ap-calculus-review-implicit-variation\/","title":{"rendered":"ap calculus review: implicit variation"},"content":{"rendered":"
implicit variation<\/strong> (or implicit differentiation<\/strong>) is a powerful technique for finding derivatives of certain equations. in this review article, we’ll see how to use the method of implicit variation on ap calculus problems.<\/p>\n the usual differentiation rules, such as power rule<\/em>, chain rule<\/em>, and the others, apply only to functions of the form y<\/em> = f<\/em>(x<\/em>). in other words, you have to start with a function f<\/em> that is written only in terms of the variable x<\/em>.<\/p>\n but what if you want to know the slope at a point on a circle whose equation is x<\/em>2<\/sup> + y<\/em>2<\/sup> = 16, for example?<\/p>\n here, it would be possible to solve the equation for y<\/em> and then proceed to take a derivative. however, that’s not really the best way!<\/p>\n for one thing, that square root will make finding the derivative more challenging. <\/p>\n for another, you really have two separate<\/em> functions — one using the plus (+), and the other using minus (-) in front of the radical! which one should you use for finding the derivative? well, that depends on whether you want the top or bottom semicircle.<\/p>\n it would be so much simpler to just work with the original equation (x<\/em>2<\/sup> + y<\/em>2<\/sup> = 16 in this discussion) rather than to solve it out for y<\/em>.<\/p>\n well we’re in luck! the method of implicit variation does exactly that!<\/p>\n given:<\/strong> an equation involving both x<\/em> and y<\/em>.<\/p>\n goal:<\/strong> to find an expression for the derivative, dy<\/em>\/dx<\/em>.<\/p>\n method:<\/strong><\/p>\n basically, whenever you take a derivative of a term involving y<\/em>, then you must tack on dy<\/em>\/dx<\/em>.\n<\/li>\n keep in mind, your final answer may involve both x<\/em> and y<\/em>.<\/p>\n<\/ol>\n this is something that had bugged me for a long time after first learning the method myself. why do we have to tack on an “extra” dy<\/em>\/dx<\/em> when taking derivatives involving y<\/em>?<\/p>\n the big idea here is that y<\/em> is actually a function. we just have no idea what that function is!<\/p>\n in a typical (or explicit<\/strong>) function, such as y<\/em> = x<\/em>3<\/sup> – 3x<\/em> + 2, y<\/em> has already been isolated. in this example, we know that the function is f<\/em>(x<\/em>) = x<\/em>3<\/sup> – 3x<\/em> + 2.<\/p>\n however, an implicit equation has not been solved for y<\/em>. in fact, it may be impossible to do so! <\/p>\n so we do the next best thing, which is simply to use our rules of calculus, including the chain rule<\/a>, whenever we encounter the unknown function y<\/em> in our equation.<\/p>\n that’s where the extra dy<\/em>\/dx<\/em> comes from. there is a hidden chain rule lurking in the background!<\/p>\n consider the equation x<\/em>2<\/sup> – 2xy<\/em> + 4y<\/em>2<\/sup> = 52.<\/p>\n (a) write an expression for the slope of the curve at any point (x<\/em>, y<\/em>).<\/p>\n (b) find the equations of the tangent lines to the curve at the point x<\/em> = 2.<\/p>\n (c) find the keyword slope<\/em> indicates that we must find a derivative<\/em>. it would be way too difficult to solve the equation explicitly<\/em> for y. so this is a job for implicit differentiation!<\/p>\n first, apply d<\/em>\/dx<\/em> to both sides.<\/p>\n the next few steps are just working out the derivatives. perhaps the trickiest part is the term involving 2xy<\/em>. think of that as the product of 2x<\/em> with an unknown function y<\/em> = f<\/em>(x<\/em>). that way, it may make more sense why we must use the product rule<\/a> for that term.<\/p>\n finally, solve for the unknown derivative algebraically. don’t forget to group<\/em>, factor<\/em> and divide<\/em>!<\/p>\n we can factor out a common factor of 2 on top and bottom to get a final answer:<\/p>\n there’s a clue in the word lines<\/em>. you should expect there to be more than one answer.<\/p>\n first find the y<\/em>-coordinate(s) that correspond to x<\/em> = 2. we do this by plugging x<\/em> = 2 into the original equation.<\/p>\n so there are two solutions: y<\/em> = 4 and y<\/em> = -3. this means that there are two different points at which we must find a tangent line. at each point, plug in the (x<\/em>, y<\/em>) pair into dy<\/em>\/dx<\/em> from part (a) to find the slope.<\/p>\n point 1:<\/strong> (2, 4). slope = therefore, using point-slope form<\/strong> for the line, we get y<\/em> = (1\/7)(x<\/em> – 2) + 4.<\/p>\n point 2:<\/strong> (2, -3). slope = again using point-slope form, we find y<\/em> = (5\/14)(x<\/em> – 2) – 3.<\/p>\n to find the second derivative of an implicit function, you must take a derivative of the first derivative (of course!).<\/p>\nwhat is implicit variation?<\/h2>\n
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an easier way<\/h3>\n
the method of implicit variation (differentiation)<\/h3>\n
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where do those dy<\/em>\/dx<\/em> factors come from?<\/h3>\n
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example — free response<\/h2>\n
at (0, √(13)).<\/p>\n
(a) slope and the derivative<\/h3>\n
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(b) finding the tangent lines<\/h3>\n
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(c) implicit second derivatives<\/h3>\n